I'm getting ready this fall to divide some as well...so would also be interested in this question... I did find this article but I like having people with hands on experience like on here to give more tips http://www.nhhostas.com/dividing-hostas.htm
Hostas are remarkably resilient when it comes to division. You can really do it any time of year as long as you can keep the plants well-watered after doing so, but it's probably easiest in the spring just when the leaves start to emerge. That way you can see them but they don't get in the way. The ground can be easier to dig then as well. Try to get out the entire plant, then slice through the plant vertically, dividing it into sections. If you can't do this with a spade, break out a knife or even a saw, which is what I've had to do on occasion.